There are a few real common mistakes that everyone makes but are quite 'easy' to fix.
The first one is feet placement. Always walk with your feet traight forward. This is better for your knees your ankles and your hip. At the same time bringing all those in a better position will increasy your spine position. Also it will be harder to to sloach when your feet are facing forward.
Activation of your posterior chain. The activation of your posterior chain kind of forces you to keep a (more) neutral spine position. When i walk with a backpack i really notice this myself. When my posterior chain is lose and not supporting me my back goes into a bad position and my chest just wants to go down.
This are just two points that i work on the increase my own walkig/sitting and all other moving.
In the military we were always told to stand proud which means to keep your head up high. Trust your feet and keep your head and chin up. You'll look more confident too.
If you want to deadlift bags put the bag upright so you can keep your back straigt and the shoulders behind the weight.
From your current picture I can correct your posture in one suggestion: date a taller man.
I am kidding, but I believe that taller people have more of a tendency to slouch because we have to look down during a conversation (or slow dancing.)